High electron mobility transistor and method of manufacturing the same

ABSTRACT

A high electron mobility transistor comprises a GaN-based electron accumulation layer formed on a substrate, an electron supply layer formed on the electron accumulation layer, a source electrode and a drain electrode formed on the electron supply layer and spaced from each other, a gate electrode formed on the electron supply layer between the source and drain electrodes, and a hole absorption electrode formed on the electron accumulation layer so as to be substantially spaced from the electron supply layer. Since the hole absorption electrode is formed on the electron absorption layer in order to prevent holes generated by impact ionization from being accumulated on the electron accumulation layer, a kink phenomenon is prevented. Good drain-current/voltage characteristics are therefore obtained. A high power/high electron mobility transistor is provided with a high power-added efficiency and good linearity.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2000-094574, filed Mar.30, 2000, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a high electron mobilitytransistor (HEMT), more particularly to a GaN-based HEMT.

[0003] It is strongly expected that a nitrogen-compound field-effecttransistor using GaN serves as a power element to be operated at a highpower and at a high frequency. The nitrogen-compound field-effecttransistors which have been proposed are a Schottky gate field-effecttransistor, MESFET (metal semiconductor field-effect transistor), HEMTor MODFET (modulated doped field-effect transistor), and MISFET (metalinsulator semiconductor field-effect transistor). Of them, a GaN-basedHEMT employing Al_(x)Ga_((1-X))N as an electron supply layer isconsidered as a promising high power element since an electronconcentration can be rendered higher than that of the GaAs-based HEMT.However, a conventional GaN-based HEMT has a problem in that a kinkphenomenon sometimes occurs in the drain-current/voltagecharacteristics. If the kink phenomenon occurs, a power-added efficiencydecreases in a large signal operation performed at a high frequency. Thepower-added efficiency η is defined as η=(Pout-Pin)/VdId, wherein Poutis an output power, Pin is an input power, Vd is a supply voltage and Idis a drain current. In addition, the distortion increases and thelinearity deteriorates.

[0004] Now, the reason why the kink phenomenon occurs in the GaN-basedHEMT will be explained. FIG. 1 is a schematic cross-sectional view ofthe GaN-based HEMT according to a first conventional example. In FIG. 1,reference numerals 11, 12, 13, 14, and 15 denote a GaN electronaccumulation layer, Al_(x)Ga_((1-x))N spacer layer, n-typeAl_(x)Ga_((1-x))N electron supply layer, Al_(x)Ga_((1-x))N cap layer,and a sapphire substrate, respectively. Furthermore, a gate electrode 16is formed on the cap layer 14, while a source electrode 17 and a drainelectrode 18 are formed on the electron supply layer 13.

[0005] In the GaN-based HEMT according to the first conventionalexample, when a drain voltage increases to raise the intensity of theelectric field within the electron accumulation layer 11, a current ofelectrons flows through a strong electric field region between thesource electrode 17 and the drain electrode 18. As a result, pairs 22 ofelectrons and holes are generated by impact ionization within theelectron accumulation layer 11. The electrons thus generated flow intothe drain electrode 18, increasing the drain current a little. However,the effect of the increased drain current is small. On the other hand,the generated holes 23 are accumulated in a lower portion of theelectron accumulating layer 11 as shown in the figure, due to theabsence of the electrode for absorbing the holes. The potential of theelectron accumulation layer therefore decreases, with the result thatthe drain current substantially increases in a drain-current saturationregion of a graph showing the drain current/voltage characteristics. Thedrain current significantly increased in this way causes the kinkphenomenon shown in FIG. 2.

[0006]FIG. 3 is a schematic cross-sectional view of a GaAs-based HEMTaccording to a second conventional example.

[0007] Reference numerals 11′, 12′, 13′, 14′, and 15′ of FIG. 3 are aGaAs electron accumulation layer, Al_(x)Ga_((1-X))As spacer layer,n-type Al_(x)Ga_((1-x))As electron supply layer, Al_(x)Ga_((1-x))As caplayer, and GaAs substrate, respectively. Furthermore, a gate electrode16′ is formed on the cap layer 14′, while a source electrode 17′ and adrain electrode 18′ are formed on the electron supply layer 13′.

[0008] In the GaAs-based HEMT according to the second conventionalexample pairs 22 of electrons and holes are also generated in theelectron accumulation layer 11′ by the impact ionization as described inthe first conventional example. However, most of the holes are absorbedby the gate electrode as shown in FIG. 3. Therefore, the holes are notaccumulated in the electron accumulation layer 11′. As a result, thekink phenomenon, a problem of the GaN-based HEMT of the firstconventional example, does not occur in the GaAs-based HEMT in thesecond conventional example.

[0009] The big difference of the GaN-based HEMT of the firstconventional example from the GaAs-based HEMT of the second conventionalexample resides in that a large amount of piezoelectric polarizationcharges 21 are generated in a hetero-junction interface in the formerGaN-based HEMT. This is because the ratio between GaN andAl_(x)Ga_((1-x))N in lattice constant is larger than that between GaAsand Al_(x)Ga_((1-x))As by an order of magnitude.

[0010] When the hetero junction of the GaN layer and the AlGaN layer isformed, positive charges are accumulated in the AlGaN layer near theinterface at a GaN-layer side, whereas negative charges are accumulatedin the AlGaN layer near the interface at a gate-electrode side due tothe piezoelectric polarization effect. As a result, most of the holesgenerated by the impact ionization are prevented from flowing into thegate electrode by the piezoelectric polarization charges (positivecharges) accumulated in the AlGaN layer near the interface at the GaNlayer side. The holes are therefore accumulated in the GaN electronaccumulation layer, causing the kink phenomenon.

BRIEF SUMMARY OF THE INVENTION

[0011] An object of the present invention is to provide acompound-semiconductor-based high electron mobility transistor whilepreventing a kink phenomenon.

[0012] To attain the aforementioned object, the first aspect of thepresent invention provides a high electron mobility transistorcomprising:

[0013] a GaN-based electron accumulation layer formed on a substrate;

[0014] an electron supply layer formed on the electron accumulationlayer;

[0015] a source electrode and a drain electrode formed on the electronsupply layer and spaced from each other;

[0016] a gate electrode formed on the electron supply layer between thesource and the drain electrode; and

[0017] a hole absorption electrode formed on the electron accumulationlayer so as to be substantially spaced from the electron supply layer.

[0018] According to a second aspect of the present invention, there isprovided a high electron mobility transistor comprising:

[0019] an electron accumulation layer formed on a substrate;

[0020] an electron supply layer formed on the electron accumulationlayer, for generating a piezoelectric polarization charge of 1×10⁻⁷C/cm² or more between the electron accumulating layer and the electronsupply layer;

[0021] a source electrode and a drain electrode formed on the electronsupply layer and spaced from each other;

[0022] a gate electrode formed on the electron supply layer between thesource and the drain electrode; and

[0023] a hole absorption electrode formed on the electron accumulationlayer so as to be substantially spaced from the electron supply layer.

[0024] In the high electron mobility transistor, the hole absorptionelectrode, which is substantially isolated from the electron supplylayer, may be formed spaced apart from the electron supply layer in sucha manner that the hole absorption electrode is not electrically affectedby the electron supply layer. However, it is preferable that the holeabsorption electrode is completely isolated from the electron supplylayer.

[0025] The high electron mobility transistor is preferably constitutedas follows.

[0026] (1) The hole absorption electrode is formed on the electronaccumulation layer via a semiconductor layer having a smaller bandgapwidth than that of the electron accumulation layer.

[0027] (2) The hole absorption electrode is formed on the electronaccumulation layer via a p-type semiconductor layer.

[0028] (3) The hole absorption electrode is formed of the same materialas the gate electrode.

[0029] (4) The source electrode is formed between the hole absorptionelectrode and the gate electrode.

[0030] (5) The hole absorption electrode is formed in parallel with thegate electrode in a gate width direction and has substantially the samelength as that of the source electrode in the gate width direction.

[0031] According to a third aspect of the present invention, there isprovided a method of manufacturing a high electron mobility transistor,comprising

[0032] a first step of laminating an electron accumulation layer and anelectron supply layer successively on a substrate;

[0033] a second step of selectively removing the electron supply layerto isolate an element region;

[0034] a third step of forming a source and a drain electrode on theelectron supply layer of the isolated element region; and

[0035] a fourth step of forming a hole absorption electrode on theelement accumulation layer exposed by the selective removal of theelectron supply layer, and simultaneously forming a gate electrode onthe electron supply layer of the isolated element region.

[0036] According to the present invention, since the hole absorptionelectrode is formed on the electron accumulation layer in order toprevent holes generated by impact ionization from being accumulated onthe electron accumulation layer, a kink phenomenon can be prevented. Asa result, a good drain current/voltage characteristics can be obtained.It is therefore possible to obtain a high power/high electron mobilitytransistor having a good linearity and a high power-added efficiency.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0037]FIG. 1 is a schematic cross-sectional view of a GaN-based highelectron mobility transistor according to a first conventional example;

[0038]FIG. 2 is a graph showing typical drain-voltage/currentcharacteristics of a high electron mobility transistor having a kinkphenomenon caused therein;

[0039]FIG. 3 is a schematic cross-sectional view of a GaAs-based highelectron mobility transistor according to a second conventional example;

[0040]FIG. 4 is a schematic cross-sectional view of a high electronmobility transistor according to a first embodiment of the presentinvention;

[0041]FIG. 5 is a schematic cross-sectional view of a high electronmobility transistor according to a second embodiment of the presentinvention; and

[0042]FIG. 6 is a schematic top view of a high electron mobilitytransistor according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0043] Now, embodiments of the present invention will be explained withreference to the accompanying drawings.

[0044] (First Embodiment)

[0045]FIG. 4 is a schematic cross-sectional view of a high electronmobility transistor (HEMT) according to a first embodiment of thepresent invention. Reference numerals 11, 12, 13, 14, and 15 denote aGaN electron accumulation layer, Al_(x)Ga_((1-x))N spacer layer, n-typeAl_(x)Ga_((1-x))N electron supply layer, Al_(x)Ga_((1-x))N cap layer,and sapphire substrate, respectively. A gate electrode 16 is formed onthe cap layer 14, while a source electrode 17 and a drain electrode 18are formed on the electron supply layer 13. Furthermore, a holeabsorption electrode 19 is formed for absorbing holes in a recessportion 24. The recess portion 24 is formed for isolation by removing aperipheral potion, other than an element region, of layers 12, 13 and 14to reach the electron accumulation layer 11.

[0046] A manufacturing method of the HEMT of the first embodiment isdescribed below. The undoped GaN electron accumulation layer 11 of 2 μmthick is grown on the (0001) sapphire substrate 15 by a metal organicchemical vapor deposition (MOCVD method). On the electron accumulationlayer 11, an undoped Al_(0.3)Ga_(0.7)N spacer layer 12 of 10 nm, andthen, an n-type Al_(0.3)Ga_(0.7)N electron supply layer 13 of 10 nm areformed successively by the same MOCVD method. The electron supply layer13 contains Si as an impurity in a donor concentration of 4×10¹⁸ cm⁻³.On the electron supply layer 13, the undoped Al_(0.3)Ga_(0.7)N cap layer14 of 5 nm is formed. Next, a first etching for isolation is performedto form an element region in the form of a mesa, thereby exposing theelectron accumulation layer 11 corresponding to the isolation region 24.Thereafter, a second etching is applied to the cap layer 14 to exposethe portion of the electron supply layer 13 which is to be allowed intocontact with an ohmic electrode to be formed later.

[0047] Subsequently, electrode layers 17 and 18 are formed as a sourceand a drain electrode (ohmic electrodes), by depositing and laminatingTi, Al, Ti and Au in this order from the bottom on the electron supplylayer 13 exposed by the second etching, followed by subjecting to a heattreatment at 900° C. for 30 seconds. Next, as a Schottky gate electrode,the gate electrode 16 is formed on the cap layer 14 by depositing andlaminating Pt, Ti and Au in this order from the bottom. Furthermore, inthe electron accumulation layer 11 exposed in the previous process, inother words, in the bottom surface of the recess portion 24 formed forisolation, a hole absorption electrode 19 is formed by depositing andlaminating Ni and Au, or Pt, Ti, Pt and Au in this order from thebottom. These materials are selected as electrode materials capable ofohmic contact for holes of the electron accumulation layer 11.

[0048] In the first embodiment, the hole absorption electrode 19 isformed to be in ohmic contact for holes of the GaN electron accumulationlayer 11. However, as a large current is not expected to flow forabsorbing holes, an electrode material to form a Schottky barrier withthe undoped GaN electron accumulation layer 11 may be selected for thehole absorption electrode 19. When the Schottky junction electrode isused as the hole absorption electrode, the ohmic electrodes for sourceand drain electrodes may be formed at first, and, thereafter, theSchottky electrode is formed simultaneously with the gate electrode, bya deposition method.

[0049] A field-effect transistor having a gate length of 1 μm was formedin the same construction manner as above. Thereafter, powercharacteristics were measured by setting the voltage of the holeabsorption electrode 19 at the same potential as the source electrode 17or lower, depending upon the operation point, in order to absorb theholes. As a result, the maximum value of the power-added efficiencyincreased by 5% compared to the conventional structure shown in FIG. 1.In addition, as the third-order intermodulation distortion was measuredat the same output power, it decreased by 10 dBc than that of theconventional structure. Therefore, it was confirmed that the powercharacteristics show good linearity.

[0050] The reason why the power characteristics are improved is that thekink phenomenon shown in a drain current/voltage characteristics(explained in the first conventional example) is suppressed by thepresence of the hole absorption electrode 19.

[0051] In the device structure according to the first embodiment, holesof pairs 22 of electrons and holes generated by impact ionization arequickly absorbed by the hole absorption electrode 19 and therefore notaccumulated in the electron accumulation layer. Therefore, the potentialof the electron accumulation layer can be stabilized, suppressing thekink phenomenon. As a result, it is possible to provide ahigh-performance device high in power-added efficiency and low indistortion.

[0052] The potential for the holes is lower at a side of the sourceelectrode 17. Therefore, the holes generated by impact ionization areaccumulated at the electron accumulation layer near the side of thesource electrode 17. In this case, if the hole absorption electrode 19is formed near the source electrode 17, as shown in FIG. 4, the holescan be efficiently absorbed.

[0053] In the first embodiment explained above, GaN is used as theelectron accumulation layer 11 and AlGaN is used as the electron supplylayer 13. However, the present invention can be effectively applied toany combination of semiconductor materials employed as the electronaccumulation layer 11 and the electron supply layer 13, as long as apiezoelectric polarization charge 21 of 1×10⁻⁷ C/cm² or more isgenerated by lattice mismatch near the hetero junction interface betweenboth layers 11 and 13. Note that a piezoelectric polarization charge ofabout 4.6×10⁻⁷ C/cm² is generated between Al_(0.1)Ga_(0.9)N and GaNlayers.

[0054] (Second Embodiment)

[0055]FIG. 5 is a schematic cross-sectional view of a high electronmobility transistor according to a second embodiment of the presentinvention. The feature of the second embodiment resides in that a p-typesemiconductor layer 20 (e.g., p-type GaN layer), or a semiconductorlayer having a smaller bandgap width than that of the electronaccumulation layer 11 is formed on the undoped GaN electron accumulationlayer 11, and thereafter, a hole absorption electrode 19 is formed onthe semiconductor layer 20. In this manner, it is possible to absorbholes more effectively than in the first embodiment. In FIG. 5, likereference numerals are used to designate like structural elementscorresponding to those in FIG. 4 (the first example) and any furtherexplanation is omitted for brevity's sake.

[0056] (Third Embodiment)

[0057]FIG. 6 is a schematic top view of a high electron mobilitytransistor according to a third embodiment of the present invention. Thefirst HEMT electrodes are formed of a gate electrode 16-1, a sourceelectrode 17-1, and a drain electrode 18-1. The second HEMT electrodesare formed of a gate electrode 16-2, a source electrode 17-2, and adrain electrode 18-2. The feature of the third embodiment resides inthat the hole absorption electrode 19 is formed in parallel with thegate electrode 16-1 in the gate width direction and has substantiallythe same length as the source electrode 17-1 in the gate widthdirection. Since the length of the hole absorption electrode 19 is thesame as that of the source electrode 17-1, the effect of the holeabsorption can be made uniform at any cross section of the drain currentdirection within the FET, and the kink phenomenon is most effectivelysuppressed compared to the case where the hole absorption electrode isshorter than the source electrode. Furthermore, the holes from the twoHEMTs can be absorbed by a single hole absorption electrode 19, so thatthe layout area of the elements can be effectively reduced.

[0058] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A high electron mobility transistor comprising: aGaN-based electron accumulation layer formed on a substrate; an electronsupply layer formed on the electron accumulation layer; a sourceelectrode and a drain electrode formed on the electron supply layer andspaced from each other; a gate electrode formed on the electron supplylayer between the source and the drain electrode; and a hole absorptionelectrode formed on the electron accumulation layer so as to besubstantially spaced from the electron supply layer.
 2. The highelectron mobility transistor according to claim 1 , wherein the holeabsorption electrode is formed on the electron accumulation layer via asemiconductor layer having a smaller bandgap width than that of theelectron accumulation layer.
 3. The high electron mobility transistoraccording to claim 1 , wherein the hole absorption electrode is formedon the electron accumulation layer via a p-type semiconductor layer. 4.The high electron mobility transistor according to claim 1 , wherein thehole absorption electrode is formed of the same material as used in thegate electrode.
 5. The high electron mobility transistor according toclaim 1 , wherein a composition of the electron supply layer is AlGaN.6. The high electron mobility transistor according to claim 1 , whereinthe source electrode is formed between the hole absorption electrode andthe gate electrode.
 7. The high electron mobility transistor accordingto claim 1 , wherein the hole absorption electrode is formed in parallelwith the gate electrode in a gate width direction and havingsubstantially the same length as that of the source electrode in thegate width direction.
 8. A high electron mobility transistor comprising:an electron accumulation layer formed on a substrate; an electron supplylayer formed on the electron accumulation layer and generating apiezoelectric polarization charge of 1×10⁻⁷ C/Cm² between the electronaccumulating layer and the electron supply layer; a source electrode anda drain electrode formed on the electron supply layer and spaced formeach other; a gate electrode formed on the electron supply layer betweenthe source and the drain electrode; and a hole absorption electrodeformed on the electron accumulation layer so as to substantially spacedfrom the electron supply layer.
 9. The high electron mobility transistoraccording to claim 8 , wherein the hole absorption electrode is formedon the electron accumulation layer via a semiconductor layer having asmaller bandgap width than that of the electron accumulation layer. 10.The high electron mobility transistor according to claim 8 , wherein thehole absorption electrode is formed on the electron accumulation layervia a p-type semiconductor layer.
 11. The high electron mobilitytransistor according to claim 8 , wherein the hole absorption electrodeis formed of the same material as that of the gate electrode.
 12. Thehigh electron mobility transistor according to claim 8 , wherein thesource electrode is formed between the hole absorption electrode and thegate electrode.
 13. The high electron mobility transistor according toclaim 12 , wherein the hole absorption electrode is formed in parallelwith the gate electrode in a gate width direction and has substantiallythe same length as that of the source electrode in the gate widthdirection.
 14. A method of manufacturing a high electron mobilitytransistor, comprising a first step of laminating an electronaccumulation layer and an electron supply layer successively on asubstrate; a second step of selectively removing the electron supplylayer to isolate an element region; a third step of forming a source anda drain electrode on the electron supply layer of the isolated elementregion; and a fourth step of forming a hole absorption electrode on theelement accumulation layer exposed by the selective removal of theelectron supply layer, and simultaneously forming a gate electrode onthe electron supply layer of the isolated element region.
 15. The methodof manufacturing a high electron mobility transistor according to claim14 , wherein the fourth step includes a step of forming the holeabsorption electrode on the electron accumulation layer via asemiconductor layer having a smaller bandgap width than that of theelement accumulation layer.
 16. The method of manufacturing a highelectron mobility transistor according to claim 14 , wherein the fourthstep includes a step of forming the hole absorption electrode on theelectron accumulation layer via a p-type semiconductor layer.
 17. Themethod of manufacturing a high electron mobility transistor according toclaim 14 , wherein the fourth step includes a step of forming the holeabsorption electrode adjacent to the source electrode.
 18. The method offorming a high electron mobility transistor according to claim 17 ,wherein the fourth step includes a step of forming the hole absorptionelectrode in parallel with the gate electrode in a gate width directionand having the substantially the same length as that of the sourceelectrode in the gate width direction.